Concise Bible Dictionary:
This was not a form of capital punishment in the Old Testament. Ishbosheth was beheaded by his murderers that his head might be carried to David (2 Sam. 4:7-8); as Goliath’s head had been carried to Saul. In the New Testament John the Baptist was killed in the Roman manner of beheading with the sword (Matt. 14:10; Mark 6:16, 27; Luke 9:9). In Revelation 20:4, those “beheaded” for the witness of Jesus, may be killed in other ways, for the word πελεκίζω signifies “to cut with an ax,”‘ having no particular reference to the head.
Strong’s Dictionary of Greek Words:
Transliteration:
pelekizo
Meaning:
from a derivative of 4141 (meaning an axe); to chop off (the head), i.e. truncate